The present invention relates to data recovery systems and, more particularly, to circuitry for reducing errors in data recovered from a double frequency encoded signal.
Data recovery systems, including such systems for use in recovering data from mass storage devices, are well-known in the art. Mass storage devices may use magnetic tapes or discs upon which data is stored in the form of magnetic pulses. To permit the storage of data on such tapes or discs, techniques such as double frequency encoding are employed. In double frequency encoding, as well as in many other conventional encoding techniques, the magnetic pulses recorded as a signal on the magnetic medium represent both data information and synchronizing or clocking information. Accordingly, when data is to be stored, a data signal and a clocking signal are encoded together into a single signal on the medium. When data is retrieved, the encoded signal must be decoded into recovered data and clocking signals, and then the data signal properly synchronized with the clocking signal.
While many commercially available mass memory or storage units can rapidly encode and store data and clocking information, and then retrieve the data and decode it when the data is needed by a data processing system, difficulties will occasionally result from errors occurring during the synchronizing of the recovered data signal with the recovered clocking signal.
For example, in the Model No. 9427H ten-mega-byte cartridge disc drive sold by Magnetic Peripherals Inc., Oklahoma City, Okla., when data is retrieved from a magnetic disc it is provided in the form of a separated data signal that represents the values of the data stored on the disc and in the form of a separated clock or clocking signal that is used to synchronize the separated data signal. Since the data on the magnetic disc is encoded in a double frequency code, the resulting separated data signal will normally represent data only in the second half of each bit cycle of the recovered signal. The second half of each bit cycle is often referred to as the "data window" or "data window portion" of the bit cycle. If the separated clock signal is slightly out of phase, it may synchronize the separated data signal at a time other than during the data window of one of the bit cycles, and result in an error in the recovered data.
There has arisen, therefore, the need for a data recovery system in which data errors will not occur as a result of the separated clock signal being misaligned with the data window of the separated data signal.